Corundum is a crystalline form of aluminium oxide (Al
2
O
3
) typically containing traces of iron, titanium, vanadium and chromium. It is a rock-forming mineral. It is a naturally transparent material, but can have different colors when impurities are present. Transparent specimens are used as gems, called "ruby" if red and "padparadscha" if pink-orange. All other colors are called "sapphire", e.g., "green sapphire" for a green specimen. MORE
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